This invention relates generally to composite materials characterized by a controlled coefficient of thermal expansion and, more particularly, to such a composite material which may be fabricated to have any selected thermal coefficient within a prescribed coefficient range. The invention relates also to a circuit board constructed of the composite material, whereby integrated circuit chip carriers can be directly mounted on the board without differential expansion problems.
According to its broader aspects, the invention provides a composite material of the class described which is adapted for use in any application in which the controlled thermal coefficient of the material is useful. The primary application of the composite material, however, is printed circuit boards, and for this reason the invention will be described in the context of such circuit boards.
For many years, integrated circuit chips were packaged almost exclusively in the form of flat-pack packages or dual in-line packages, known as DIPs. A dual in-line package has two rows of leads that can be inserted in holes for soldering to circuit boards. In recent years, the use of much smaller chip carriers has emerged as a more desirable packaging technique for integrated circuits. Chip carriers are usually fabricated from a ceramic material, such as aluminum oxide, and produced as a hermetically sealed package for each chip. Bonded leads are brought out from the chip to the edges of the chip carrier, and ideally the carrier is then soldered, by its leads, directly to a circuit board. The principal advantage of this structure is a significantly higher circuit density. Also, shorter and more uniform lead lengths result in improved speed and impedence characteristics. Furthermore, use of chip carriers substantially reduces the overall cost of a circuit package. Package size reductions as high as five-to-one ratio can be obtained, compared with an equivalent dual in-line package construction.
A major drawback to the use of chip carriers in that the coefficient of thermal expansion of aluminum oxide, the most commonly used ceramic chip carrier material, is approximately one-half the coefficient of thermal expansion for glass/epoxy laminates typically used in the manufacture of circuit boards. When the resulting structure is exposed to any significant range of temperatures, the thermal cycling of the structure can crack soldered joints and render the circuit inoperative. One solution to this problem is to place between the chip carrier and the circuit board and intermediate member which accommodates relative thermal expansion and contraction of the chip carriers and circuit board. The circuit board is sometimes referred to as a mother board and the intermediate member as a baby board. The intermediate member may also take the form of a hybrid package on which the chip carrier is mounted. Another technique is to use a compliant lead structure between the chip carrier and the circuit board, although this clearly increases the cost of the package and results in inherently long lead lengths.
It will be appreciated from the foregoing that there has been a significant need for a new technique that reduces or eliminates this problem of mismatch between the coefficients of thermal expansion of chip carriers and the circuit boards on which they are mounted. The present invention satisfies this need.